I have a USB NAS device which is used for daily backups on Windows Server 2003. Now if I plug the USB device in, the server detects it and I can use it normally but after a day or so, the NAS device cant been seen. The only way to get it recognised again is to unplug it and then plug it back it. I can plug it back into the same USB port or a different port and it will see the device.
Which USB NAS device is this? I'm assuming that it shuts itself off after a set duration, causing your problem. Have you checked the device's settings?